BER Analysis for Uplink NOMA in Asymmetric Channels

The bit error rate (BER) analysis has been recognized as an effective approach to investigate the problems in non-orthogonal multiple access (NOMA) systems, such as inter-user interference (IUI) and error propagation. However, the impact of asymmetric channels on adaptive modulation and coding (AMC) will bring great challenges to the BER analysis of NOMA systems. In this letter, by exploiting the scaling characteristics of Euclidean distance, we disclose the boundary effect of asymmetric channels and derive the boundary value for paired users. In addition, we provide closed-form BER expressions for uplink NOMA users on both sides of the boundary value. Numerical results verify the existence of the boundary value and conform to our derivations.